Substances differ in their ability to absorb or reflect light of certain wavelengths. For example, metallized surfaces of semiconductor wafers are highly reflective in comparison with non-metallized silicon surfaces. Silicon surfaces tend to admit rather than reflect light, particularly light in the infrared range. Optically discernible differences can be transduced into electrically differentiated output signals in opto-electronic measuring and counting techniques.
For example, a publication by C. R. Yeich, Jr., Technical Digest No. 35, p. 39, 1975, Western Electric Co., discloses in relation to processing semiconductor devices, such as light emitting diodes (LEDs), opto-electronic scanning to count the number of discrete chips in an array. A TV camera is employed to view the articles and to provide a TV image of the array on a monitor screen. Certain video scan lines are selected by a line selector circuit to generate a spacing between the scan lines, such that each row of chips in the array is scanned only once. The chips are automatically counted by a counter which simply sums up the number of video pulses, one of which is generated each time a scan line passes over the image of one of the chips.
While the chips to be counted are still arranged in a close array of the original wafer, such that substantially no gaps exist between the rows and columns of the chips, the above counting technique may fail when the gaps between the rows and columns are no longer recognizable. However, other opto-electronic techniques are available, whereby surface areas which are either darker or brighter than a background or threshold level are identified and measured. According to a particular opto-electronic measuring technique, uniformly timed pulses are produced in response to a coincidence between a video signal of an image of the area to be measured and a special raster scan signal. The pulses are counted to become an indication of the size of the area. The pulse count may be calibrated, for example, by comparing the pulse count of the measured area with a pulse count of a similar area of known size.
It is readily seen that for such a measuring technique to be applied for scanning and measuring the area of chips, the boundaries of images of the chips should be sufficiently distinct over a surrounding background image to be correctly identified. Not only should the boundaries of the chips or area to be measured be distinct over such a surrounding background, but also the entire area to be measured should be of an optically distinct, reflective or absorptive value over that of the surrounding background.
Variations in a uniform brightness value occur at times over a larger area, apparently because of topographical irregularities within such area which tend to cause reflected light to scatter nonuniformly. Such topographical irregularities tend to occur in semiconductor chip arrays when a few single chips or entire groups of chips become unevenly embedded in a mounting substance, such as mounting wax, which holds the chips on a substrate after chips have been cut from a wafer. The uneven embedding results in a waviness of the top surface of the area formed by the array of chips. It is this waviness in the flatness of surfaces to be identified which causes the variations in the uniform brightness of light reflected from such surfaces. Such variations can become so great at times that darker appearing portions of generally reflective areas fall below the threshold value which identifies, for example, the generally absorptive background. It appears whenever such variations occur, errors are introduced into, for example, the measurement of areas because portions of the areas to be measured cannot be identified by the opto-electronic techniques and are mistaken for background areas.